Oil-hydraulic vehicles have long been available. Those currently in use comprise an engine, a transmission, an oil-hydraulic pump, and oil-hydraulic motors. They transform engine power into wheel-driving power as follows.
First, the turning force of an engine is transformed by a transmission. Second, the transformed turning force is transformed into oil-hydraulic force by an oil-hydraulic pump. Then, the oil-hydraulic force is transformed into turning force by oil-hydraulic motors to drive wheels.
In this way, the turning force of an engine is transformed into wheel-driving force.
Because the transmission of an oil-hydraulic vehicle currently in use consists of a plurality of shafts and many gears, its structure is complex and it is heavy, making the whole vehicle heavy.
Besides, to change gears, it is necessary to let out the clutch between the engine and the transmission. Thus, the drive line from the engine to the oil-hydraulic pump is complex.